VENERID^. 1'75 



and ribbed, distinctly gaping posteriorly; hinge in the right 

 valve with two cardinal teeth, and a third ver^' small, but usually 

 obsolete, anterior ; the middle one is prominent, curved as in 

 Petricola ; the posterior is longitudinally lamellar, low and 

 bifurcate ; in the left valve are three distant and very unequal 

 cardinal teeth ; the middle one is similarly projecting as the 

 corresponding tooth in the other valve. Rup. lamellifera^ 

 Conrad, may be considered as a type of the group. 



Choristodon (Jonas, 1844), H. and Adams, 185t. 



Syn. — Naranio, Gray, 1853. 



Distr. — 3 sp. Polynesia, Mazatlan. C. divaricatuin, Chemn. 

 (cxii, 94). 



Shell oval-quadrangular, rugose or tuberculose, swollen, beaks 

 anterior; two cardinal teeth in each valve, the superior one of 

 the rigljt valve compressed and elongated, that of the left valve 

 oblique and bifid ; no lateral teeth ; ligament external, short, 

 in a deep groove ; anterior muscular impression oblong, poste- 

 rior one large, rounded ; pallial sinus rounded, deep, 



Saxidomus, Conrad, 188*7. 



Distr. — Californian Province. S. Nuttallii^ Conrad (cxii, 91). 



Shell transversel}^ oval, inequilateral, solid, ventricose, with- 

 out lunule, umbones tumid ; teeth three or four, unequal, narrow, 

 the central bifid ; ligament very thick, elongated ; muscular 

 impi'essions oval or rounded, nearly equal ; pallial sinus large, 

 elongated, horizontal. 



Family VENERID^. 



Shell regular, closed, suborbicular, or oblong ; ligament 

 external; hinge with usually three diverging teeth in each valve; 

 muscular impressions oval, polished; pallial line sinuated. 



Animal free, locomotive, rarely byssiferous or burrowing; 

 mantle with a rather large anterior opening ; siphons unequal, 

 united more or less ; foot linguiform, compressed, sometimes 

 grooved; palpi moderate, triangular, pointed; branchia large, 

 subquadrate, united posteriorly. 



The shells of this tribe are remarkable for the elegance of their 

 forms and colors ; they are frequently ornamented with chevron- 

 shaped lines. Their texture is very hard, all traces of structure 

 being usually obliterated. The Veneridae appeared first in the 

 Oolitic period, and have attained their greatest development at 

 the present time ; they are found in all seas, but most abund- 

 antly^ in the tropics. 



Subfamily VENERINJE. 



Shell oval or subtrigonal. Siphons free to their extremity ; 

 foot lanceolate, without byssiferous groove. 



